be running Linux with the 2.4 or 2.2 kernel series. See this
kernel compatibility table.
We also have experimental support for 2.6 kernels. There are two
basic approaches, and this document covers both:

install FreeS/WAN, including its KLIPS
kernel code. You'll be using KLIPS in place of the existing 2.6 IPsec
kernel support.

These instructions are for a recent Red Hat or Fedora Core with a
stock Red Hat or Fedora Core kernel. We know that Mandrake and SUSE
also produce FreeS/WAN RPMs. If you're running either, install using
your distribution's tools.

Andreas Steffen, author of the X.509 patch, has started his own fork
of FreeS/WAN, Strongswan, a
FreeS/WAN code fork which includes advanced X.509 support, Delete
SA/notifications, Network Address
Translation support and AES.

KLIPS install for 2.2, 2.4 and 2.6 kernels

To make a modular version of KLIPS for 2.2 and 2.4 kernels, along
with other FreeS/WAN programs you'll need, use a command sequence like
the one below. This will change to your new FreeS/WAN directory, make
the FreeS/WAN module (and other stuff), and install it all.

cd /usr/src/freeswan-2.06
make menumod # just save and exit
make minstall

To link KLIPS statically into your kernel on 2.2, 2.4 or 2.6 (using
your old kernel settings), or to build a KLIPS module for
2.6, you'll need to patch the kernel itself. The following will change
to your new FreeS/WAN directory, compile KLIPS into your kernel (and
other stuff), and install it all.

cd /usr/src/freeswan-2.06
make [KERNELSRC=/usr/src/linux-2.6.1-1.47] menugo # just save and exit
make minstall

The KERNELSRC argument is necessary for 2.6 kernels, as it defaults
to /usr/src/linux-2.4.

Checking your system to see if IPsec got installed and started correctly
Version check and ipsec on-path [OK]
Checking for KLIPS support in kernel [OK]
Checking for RSA private key (/etc/ipsec.secrets) [OK]
Checking that pluto is running [OK]